Alteration Index (CAI) is a function of the carbonate mineralization in
the soil being analyzed. CAI is a measure of the buffering capacity of
bases to neutralize acids. Measuring CAI is important in determining a
soils ability to precipitate carbon as carbonate byproducts of the
hydrocarbon seepage and soil alteration process. CAI is not pH, but
instead refers to the ability of a soils interstitial water to resist
change in pH. The presence of buffering materials help neutralize acids
as they are added to soil pore water. These buffering materials are
primarily the bases bicarbonate (HCO3-), and carbonate (CO32-) and occasionally hydroxide (OH-), borates, silicates, phosphates, ammonium, sulfides, and organic ligands.
not only helps regulate the pH of a soil, but also the metal content.
Bicarbonate and carbonate ions in water can remove toxic metals (such as
lead, arsenic, and cadmium) by precipitating the metals out of
solution. This allows many of the transition elements to be used as
hydrocarbon indicators. Siderite (FeCO3) calcite (CaCO3)
are early diagenetic indicators of oil with siderite detection being
one of the early geochemical pathfinder minerals often referred to as
the delta C carbonate.
The use of
CAI as a hydrocarbon exploration tool relies on the accumulation of
bacterial end-products in pore fluids which result in the precipitation
of carbonate minerals. These end products affect pore water pH, Eh, and